Catalytic reforming, or hydroforming, is a well-established industrial process employed by the petroleum industry for improving the octane quality of naphthas or straight run gasolines. In reforming, a multi-functional catalyst is employed which contains a metal hydrogenation-dehydrogenation (hydrogen transfer) component, or components, substantially atomically dispersed upon the surface of a porous, inorganic oxide support, notably alumina. Noble metal catalysts, notably of the platinum type, are currently employed in reforming. Platinum has been widely commercially used in recent years in the production of reforming catalysts, and platinum-on-alumina catalysts have been commercially employed in refineries for the last few decades. In the last decade, additional metallic components have been added to platinum as promotors to further improve the activity or selectivity, or both, of the basic platinum catalyst, e.g., iridium, rhenium, tin, and the like. Reforming is defined as the total effect of the molecular changes, or hydrocarbon reactions, produced by dehydrogenation of cyclohexanes and dehydroisomerization of alkylcyclopentanes to yield aromatics; dehydrogenation of paraffins to yield olefins; dehydrocyclization of paraffins and olefins to yield aromatics; isomerization of normal paraffins; isomerization of alkylcycloparaffins to yield cyclohexanes; isomerization of substituted aromatics; and hydrocracking of paraffins which produces gas, and inevitably coke, the latter being deposited on the catalyst.
In a conventional process, a series of reactors constitute the heart of the reforming unit. Each reforming reactor is generally provided with fixed beds of the catalyst which receive upflow or downflow feed, and each is provided with a heater, because the reactions which take place are endothermic. A naphtha feed, with hydrogen, or hydrogen recycle gas, is concurrently passed through a preheat furnace and reactor, and then in sequence through subsequent interstage heaters and reactors of the series. The product from the last reactor is separated into a liquid fraction, and a vaporous effluent. The latter is a gas rich in hydrogen, and usually contains small amounts of normally gaseous hydrocarbons, from which hydrogen is separated from the C.sub.5.sup.+ liquid product and recycled to the process to minimize coke production.
The activity of the catalyst gradually declines due to the buildup of coke. Coke formation is believed to result from the deposition of coke precursors such as anthracene, coronene, ovalene and other condensed ring aromatic molecules on the catalyst, these polymerizing to form coke. During operation, the temperature of the process is gradually raised to compensate for the activity loss caused by the coke deposition. Eventually, however, economics dictates the necessity of reactivating the catalyst. Consequently, in all processes of this type the catalyst must necessarily be periodically regenerated by burning the coke off the catalyst at controlled conditions, this constituting an initial phase of catalyst reactivation.
Two major types of reforming are generally practiced in the multi-reactor units, both of which necessitate periodic reactivation of the catalyst, the initial sequence of which requires regeneration, i.e., burning the coke from the catalyst. Reactivation of the catalyst is then completed in a sequence of steps wherein the agglomerated metal hydrogenation-dehydrogenation components are atomically redispersed. In the semi-regenerative process, a process of the first type, the entire unit is operated by gradually and progressively increasing the temperature to maintain the activity of the catalyst caused by the coke deposition, until finally the entire unit is shut down for regeneration, and reactivation, of the catalyst. In the second, a continuous or cyclic type of process, the reactors are individually isolated, or in effect swung out of line by various manifolding arrangements, motor operated valving and the like. The catalyst is regenerated to remove the coke deposits, and reactivated while the other reactors of the series remain on stream. A "swing reactor" temporarily replaces a reactor which is removed from the series for regeneration and reactivation of the catalyst, until it is put back in series.
There are several steps required for the regeneration, and reactivation of a catalyst. Typically, regeneration of a catalyst is accomplished in a primary and secondary coke burnoff. This is accomplished, initially, by burning the coke from the catalyst at a relatively low temperature, i.e., at about 800.degree. F.-950.degree. F., by the addition of a gas, usually nitrogen or flue gas, which contains about 0.6-1 mole percent oxygen. A characteristic of the primary burn is that essentially all of the oxygen is consumed, with essentially no oxygen being contained in the reactor gas outlet. Regeneration is carried out once-through, or by recycle of the gas to the unit. The temperature is gradually raised and maintained at about 950.degree. F. until essentially all of the coke has been burned from the catalyst, and then the oxygen concentration in the gas is increased, generally to about 6 mole percent. The main purpose of the secondary burn is to insure thorough removal of coke from the catalyst within all portions of the reactor. The catalyst is then rejuvenated with chlorine and oxygen, reduced, and then sulfided. Thus, the agglomerated metal, or metals, of the catalyst, is redispersed by contacting the catalyst with a gaseous admixture containing a sufficient amount of a chloride, e.g., carbon tetrachloride, to decompose in situ and deposit about 0.1 to about 1.5 wt. % chloride on the catalyst; continuing to add a gaseous mixture containing about 6% oxygen for a period of 2 to 4 hours while maintaining temperature of about 950.degree. F.; purging with nitrogen to remove essentially all traces of oxygen from the reactor; reducing the metals of the catalyst by contact with a hydrogen-containing gas at about 850.degree. F.; and then sulfiding the catalyst by direct contact with, e.g., a gaseous admixture of n-butyl mercaptan in hydrogen, sufficient to deposit the desired amount of sulfur on the catalyst.
In a typical continuous, or cyclic type process, the regeneration gas is generally cooled to relatively low temperatures, i.e., about 100.degree.-150.degree. F., by passage through a cool waterscrubber to remove water by condensation, and the condensate drawn off. In this step, the water produced during combustion, or desorbed from the catalyst during heatup is removed from the process. The water condensate is acidic, and highly corrosive, since it contains hydrogen chloride stripped from the catalyst. The partially dried flue gas is generally further dried by passage through a dessicant bed, or beds, prior to its return to the reactor containing the catalyst being regenerated. Excessive moisture levels are intolerable since the excessive moisture produces an acidic condensate, further facilitates chloride stripping from the catalyst, and produces high hydrogen chloride levels in the vent gas sent to the atmosphere. Thus, there is a need to prevent the moisture levels in flue gas from exceeding maximum tolerable limits, as dictated by the need to suppress, if not avoid, the amount of condensate that is formed, minimize chloride stripping, and lessen vent gas hydrogen chloride concentrations.
It is, accordingly, a primary objective of the present invention to meet this need in the operation of cyclic catalyst regeneration units, especially as used in the regeneration of noble metal reforming catalysts, more particularly platinum-containing, or platinum-containing polymetallic reforming catalysts.
A specific object is to provide a novel process for the regeneration of such catalysts, at conditions which favor high cost-effectiveness and energy efficiency by recycling as purge gas a portion of the flue gas used to remove combustion water, or water desorbed from the catalyst, while suppressing corrosion; and also minimize expensive nitrogen now used as once-through purge gas.
These objects and others are achieved in accordance with the present invention, embodying improvements in a process for regenerating, and reactivating, said types of coke deactivated noble metal catalysts, in a system which includes separate, interconnected primary and secondary regeneration gas circuits in which gas is circulated from one circuit to the other, (i) the primary regeneration gas circuit containing a preheat gas furnace, a reactor which contains said catalyst from which said coke can be burned by contact with hot gas from said preheat gas furnace, and steam boiler through which said hot gas can be passed and cooled without the occurrence of condensation, and the cool gas returned to said secondary circuit, (ii) the secondary circuit containing a regeneration gas scrubber, a gas drier (optional) and particulate filter, and including regeneration gas compression means, e.g. a compressor, for circulating the gas in said circuits. A gas, constituted in major part of flue gas, is withdrawn from the secondary circuit and passed through the preheat gas furnace, preheated, then passed into the coke catalyst-containing reactor (reaction zone) wherein the coke is combusted by contact of a combustible mixture of the gas with the catalyst, the gas heated thereby, then cooled by passage through the steam boiler, and the cooled gas then returned into the secondary circuit.
The hot flue gas, or gas within the secondary circuit, is countercurrently contacted with cooling water in a regeneration gas scrubber to condense out the combustion water and water desorbed from the catalyst, as well as extract the hydrogen chloride stripped from the catalyst. The temperature of the hot flue gas is reduced to within a range of from about 60.degree. F. to about 120.degree. F., preferably from about 80.degree. F. to about 105.degree. F., and the hydrogen chloride to a concentration ranging from about 0 to about 20, preferably from about 0 to about 5, parts per million parts by volume (vppm) of total gas, while the acid condensate is removed from the bottom of the scrubber and sent to disposal. To maintain system pressure gas is purged to the atmosphere. Air and nitrogen are added to the system as purge gas. Although the combustion air carries along a significant quantity of nitrogen, a relatively small quantity of nitrogen may be needed, and added, to prevent moisture levels from exceeding maximum tolerable limits as dictated by the need to avoid condensation, minimize chloride stripping, and the amount of hydrogen chloride vented to the atmosphere. Suitably, the gas is then dried if desired, and filtered to effect particulates removal, while the gas, as purge gas, is recycled to the primary circuit for reuse in regeneration of the deactivated catalyst. The moisture level of the gas transferred to the primary circuit is maintained at a level ranging from about 0.1 percent to about 1 percent, preferably from about 0.2 percent to about 0.6 percent, based on the total volume of gas in the secondary circuit.
In its essence, the invention is one wherein the gas is conditioned in the secondary circuit for use in the primary circuit, and constitutes a source of regeneration gas, or purge gas for use in the primary circuit. The gas is withdrawn, or transferred, from the primary circuit into the secondary circuit as needed. The increasing cost of energy has caused a significant rise in the cost of inert gas and hence, the present invention sharply reduces the need for purge gas from other sources of supply.
These features and others will be better understood by reference to the following more detailed description of the invention, and to the drawings to which reference is made.